srh@.uucp (Steve Harmon) (05/21/91)
In article <5766@mindlink.bc.ca>, nanook@eskimo.celestial.com (Robert Dinse) writes: > [request for memory upgrade info deleted] > > In order to do this you need an MMU upgrade. Stock the MMU can only >address 1 megabyte of memory. Last time I got a price quote from Tandy >it was around $300 if memory serves. I've got a couple of CPU boards with >a custom MMU (that doesn't have a PAL chip - courtesy of design work done >by Monty Schmidt) that works ok with 2-megs but not more than that. I am >speculating that it's a fan-out problem. I've been meaning to post something about MMUs for a while so I guess now is as good a time as any. Some years ago I designed, manufactured and sold MMUs for the 16B/6000. This was a year or so before Tandy released their MMU. If anyone is interested in the technical info required to build their own MMU then send me some email and I'll provide it for you. The unit is pretty simple to build (in fact the prototype was built on a piece of perf board from RS :-) and only contains 5 parts. This is probably not a very good project for people with no working knowledge of digital electronics especially since it requires modifying the CPU board. Anyway, anyone who is interested is welcome to send me some email. Also, I believe that I still have 40-50 blank MMU boards somewhere if anyone is interested (the info is free, the boards I'll probably charge for). -- | Steve Harmon @ AT&T Tridom Marietta, Georgia. | INET: srh%tridom.uucp@mathcs.emory.edu | UUCP: ..gatech!emory!tridom!srh | VOICE: (404) 426-4261 |
srh%tridom.uucp@mathcs.emory.edu (Steve Harmon) (05/28/91)
Since I've received a great deal of interest in information about
extending the MMU on the 16B/6000 I've decided to post the info. The
following should provide a little more info than my previous post.
- the MMU (as I built it) is a 3" x 5" (approx) add on board
- it is added to the CPU board in place of the CPU
- it allows up to 7 meg (actually 7.5) of User RAM under Xenix 3.2
- there are no limitations on the type of RAM boards (although some
earlier Tandy RAM boards have some interesting design problems)
- the add on is passive in a pre Xenix 3.2 system (i.e. the OS doesn't
know how to use the extended MMU and therefore is limited to the
usual 1 meg of RAM)
- there are no speed limitations for CPUs that have been modified for
greater than 8Mhz operation provided that you use fast parts in
certain critical areas when you build the board (i.e. 74F series
logic)
Via a friend I had sold over 40 of these into systems with various
configurations. The only significant memory board problem that I can remember
was related to the float (charge) time of the A19 line. I seem to recall that
this problem was on an older Tandy memory board if you tried to add over 1Meg
to the system. If there is enough interest I'll look up my notes on memory
boards and post a list of mods that will help reduce their unreliability.
BTW, if you don't have a reasonable amount of experience working on
micro-processor based equipment then I wouldn't recommend trying to add
an MMU addition to the 68000 CPU board. The CPU/memory boards designs are
pretty poor (to say the least) and they are fairly easy to break and rather
difficult to fix.
If you're reasonably handy with electronics you can put the MMU together
on a small piece of perf board (which is how I prototyped it :-) and attach
it via about 10 leads with 2 or 3 track cuts on the CPU board.
Anyway, I'll post more detailed info soon. Right now I've sent the info
to several people in order to evaluate the format of the information. One
problem that I see coming up is the format in which to distribute my schematic.
I was planning on posting it in postscript format (compressed of course), if
anyone has a better suggestion then please let me know. (Reply by email please
since I don't often read this group)
--
| Steve Harmon @ AT&T Tridom Marietta, Georgia.
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